Approximately 60 percent of Americans have refractive errors, and millions of people are myopic worldwide. Many thousands of laser refractive surgeries are performed every year for the correction of myopia. These procedures will ultimately affect a large number of individuals around the globe, and yet the corneal response to laser ablation is not well understood. Of the many individuals treated, about 15-50% do not achieve 20/20 vision which translates into very large patient numbers when the extremely high popularity of refractive surgery is considered. It is crucial that the number of patients who achieve their targeted vision be increased in order to improve the overall quality of vision in this vast group of people.
Anterior corneal surface topography cannot take into account contributions of optically important structures inside the eye, such as the posterior corneal surface and the crystalline lens. If a laser were programmed strictly with anterior topography data, the correction would be at best incomplete, and at worst simply wrong. Therefore, wavefront analysis is important, particularly if the ultimate goal is to correct higher order aberrations along with the sphere and cylinder.
Refractive errors are traditionally compensated both with ophthalmic lenses and with contact lenses. As an alternative to these correction methods, corrective surgical procedures of the incisional type, such as radial keratotomy, appeared in the '80s. They have recently been replaced by photorefractive keratectomy (PRK) and laser assisted in-situ keratomileusis (LASIK) which modify the shape of the cornea in order to thereby change its power and compensate the refractive errors. These last two procedures use an excimer laser for forming the cornea in order to remove tissue by means of ablation. While in PRK surgery the ablation commences on the surface layers of the cornea (first the epithelium and then the Bowman layer), in LASIK surgery those layers are not ablated since a microkeratome creates a surface lamina of corneal tissue which is removed prior to the ablation and replaced afterwards so that just the stroma is ablated.